Uninterruptible power supplies or systems (commonly referred to as UPS) are used to provide back-up power to critical loads such as computer systems where a loss of line power can result in the interruption of programs and the loss of valuable data. Uninterruptible power supplies may also provide a line power conditioning function to ensure that transient spikes, low-volatage conditions, or distorted power waveforms on the AC power system do not disturb the operation of the computer which is supplied with power through the UPS. Typically, the UPS includes a battery which is interfacted through an inverter to the AC output line. When a fault occurs in the input AC power, the inverter is controlled to provide power from the battery to the AC output line at the same frequency and with substantially the same waveform as the normal input AC power.
In double conversion UPS systems, the AC power is rectified to the DC voltage which is applied, in parallel with the battery voltage, to a constantly running inverter. If the line power fails, the output inverter continues to operate, but now delivers power from the back-up power source (usually a battery) to the critical load.
In UPS systems which utilize a ferroresonant transformer, the inverter need not be running constantly since power is normally delivered from the AC power mains through the ferroresonant transformer to the load, with the ferroresonant transformer providing transient filtering of input line power and some compensation of short power disturbances. Upon detection of a power outage or brownout on the AC power lines, an AC switch can be opened to disconnect the primary of the ferroresonant transformer from the AC power mains, and an inverter is turned on to supply power from a battery to an auxiliary primary of the ferroresonant transformer which takes over the job of supplying the power to the load.
In certain other types of back-up power supplies, the AC power mains are normally connected directly to the load, and an inverter is turned on to supply power to the load typically only when the AC power mains fail. An advantage of such systems is that the direct connection of the AC power lines to the load during normal operation avoids energy loss in the auxiliary power supply and allows relatively less expensive and less complicated inverter components to be used since the inverter will be operated infrequently.
In all UPS systems which use a backup battery, the length of time during which the UPS can supply usable power from its battery to the load (the "run-time") is dependent on the state of charge of the battery and the power being drawn by the load. Since battery aging changes the characteristics of the batter, and the state of charge of the battery will decrease when power is drawn from it through the inverter and will increase as the battery is recharged, the remaining run-time of the battery is a highly variable parameter. Nonetheless, it is important to proper operation of the UPS that the available run-time be accurately estimated. For example, when the estimated run-time reaches a particularly low level during UPS operation an orderly shut down of the load system (e.g., a computer) can be carried out rather than shutting down precipitously when the UPS fails for lack of battery power. It is also desirable to be able to estimate run-time when the inverter is not operating so that an operator can anticipate the length of a power outage that the UPS can handle. However, when the inverter is not running, the battery is not loaded and, in fact, it is usually being recharged, so that loaded battery voltage is not available.
One approach to determining available run-time which has been used previously in UPS which employ microprocessor control allows the run-time to be determined during normal supply of power from the AC power system by periodically disconnecting the load from the AC power system and supplying power from the battery through the inverter. Prior to carrying out the battery test, the microprocessor calculates output load power and determines a factor which will be used in the battery test. The power drawn by the load in output watts is calculated as the sum of the instantaneous product of the output voltage and output current over a cycle divided by the number of instantaneous samples acquired for a line cycle.
The UPS then switches to inverter power and measures the voltage of the battery. The battery test calculates run-time during inverter operation as follows: ##EQU1## where T.sub.IR is the estimated remaining inverter run-time available from the battery,
K is an experimentally determined constant, PA1 B.sub.c is the rated ampere hour capacity of the battery, PA1 V.sub.B is the measured DC output voltage from the battery, PA1 V.sub.LL is the predetermined low battery voltage set point at which the battery should no longer provide power, PA1 V.sub.FC is the full charge battery voltage, PA1 P.sub.LD is the calculated power drawn by the load in watts, and PA1 P.sub.LS is the loss in watts in the inverter.
The resulting run-time can be displayed to an operator or used to provide an indication if the run-time falls below a selected minimum value. As noted, this approach requires that the inverter be turned on periodically to allow the run-time to be calculated when power is being drawn from the battery. The same procedure can be used to estimate run-time when the AC power system has failed and the UPS is supplying power from the battery to the load.
U.S. Pat. No. 5,295,078 to Stich, et.al., entitled Method and Apparatus for Determination of Battery Run-Time in Uninterruptible Power System, discloses an approach to determining the available run-time on battery power in an uninterruptible power system in a highly accurate manner both during the period when power is being supplied from the battery through an inverter to the load and during normal operation where power is supplied from the AC power system to the load and the battery is being charged. In the latter case, the run-time is estimated without the need to periodically switch to inverter power to measure the battery voltage while it is supplying power to the load. In accordance with that patent, battery run-time is calculated during inverter operation using the battery terminal voltage V.sub.B measured directly from the battery. When the UPS is in line operation, however, the battery is typically being charged and V.sub.B cannot be calculated by direct measure. Instead, during the time that the battery is being charged, the open circuit battery voltage is calculated based on an initial measured opened circuit voltage, measured just prior to entering into line operation, adjusted by a factor proportional to the amount of time that the battery has been charged and based on the characteristics of the battery and the battery charger. This calculated open circuit voltage is then utilized to calculate V.sub.B under load conditions by subtracting a number proportional to the product of output current and measured battery resistance from the calculated open circuit voltage. In this way an accurate estimate of inverter battery run-time can be calculated and displayed to an operator during line operation without the necessity of periodically switching to inverter operation in order to make the calculation. Moreover, the use of an expression for run-time, which includes the sum of a term which is dependant on output current (or power) and a term that is not, more closely matches actual battery discharge characteristics than prior processes which used only a term dependant on output power.